The present invention relates to a corona discharge processing method and apparatus, and more particularly to an method and apparatus to perform surface processing by corona discharge for resin formings having various three-dimensional shapes, in order to improve properties such as coating performance, printing, adhesion and so forth.
It has been widely known that, by performing the corona discharge processing on the surface of a resin film made from polypropylene and the like, performance such as printing and adhesion on the surface of films can be improved, and this processing has been used for manufacturing various kinds of articles. The corona discharge processing is performed under ordinary pressure not only on the surface of a resin film but also on the surface of resin formings having stereoscopic unevenness, that means a three-dimensional shape, in order to improve coating performance and adhesion.
In the conventional corona discharge processing, a pair of electrodes consisting of a discharge electrode and a counter-electrode are used, an article to be processed is placed between the electrodes, corona discharge is generated by applying a high voltage between the electrodes under ordinary pressure, and thereby the corona discharge processing is performed on a surface of the article to be processed. In this performing, as a high voltage charged between the electrodes, there has been usually used an alternating-current high frequency voltage. The alternating-current high frequency voltage easily generates corona discharge, so that it is said to be superior in the effects of surface processing effect. According to the Japanese Official Patent Provisional Publication No. showa 47-4890, it has been disclosed that the processing effects are improved by using a high voltage pulse having a pulse width of from about 100 to 200 .mu.s in the corona discharge processing of films.
In the case where an article to be processed is a resin forming which has a three-dimensional shape, there have been proposed various kinds of improved methods, because it is required to perform superior corona discharge processing on a surface having unevenness and because various limitation exists compared with the case of flat films and boards.
For example, in the Japanese Official Patent Provisional Publication No. showa 57-119931, there has been disclosed a method comprising: arranging an article to be processed which is held on a counter-electrode, so that the tip of a discharge electrode always has a definite distance from the surface of the article to be processed; transferring a set of the counter-electrode and article to be processed while maintaining their relative arrangements to the discharge electrode unvariable; generating corona discharge by applying an alternating-current high frequency voltage between the counter-electrode and the discharge electrode; and accordingly, processing the surface of the article by the corona discharge processing. In the Japanese Official Patent Provisional Publication No. showa 62-57431, there has been disclosed a method comprising: relatively transferring a discharge electrode, the discharging tip of which has a curved face shape, along the surface of an article to be processed; and applying an alternating-current high frequency output of 20 to 30 kHz to perform the corona discharge processing.
However, in these kinds of corona discharge processing for resin formings having a three-dimensional shape, spark discharge easily occurs, so that there is such a problem as that sufficient processing of an edge part existing in an outer periphery of the resin formings or existing in an inner circumference of opening parts is very difficult.
This is because, in the conventional methods, when the discharge electrode approaches an edge part of the resin forming, corona discharge effective on the surface processing can not be maintained in a stable condition and spark discharge occurs by breakdown of an air layer. To avoid the spark discharge, an applied voltage may be lowered, but if doing so, the corona discharge processing does not sufficiently display the effects.
To solve these kinds of problems, in the Japanese Official Patent Provisional Publication No. showa 62-57431, there has been disclosed an art which comprises inserting a buffer board, made of a dielectric such as an epoxy resin and so forth, into an opening part of the resin forming in order to prevent a disorder of the corona discharge at the opening part. However, in this art, there needs to arrange a buffer board corresponding to a shape which an opening part of the resin forming has, and there is taken labor to fix the board at a specific position of the resin forming or counter-electrode, and therefore, there is a defect that working efficiency is very bad. Also, if there is a gap between an opening part and a buffer board, even if it is slight, breakdown occurs through this gap, which results in generation of spark discharge.
To solve the problems in prior arts as described above, the present inventors carried out intensive research on the waveform of a high voltage applied for the discharge electrode. As a result, they found that, if there is used a high voltage pulse having a very narrow pulse width, for example, such as having 10 to 100 kv in wave height, 1 .mu.s or less in pulse width and 10 to 1000 pps in pulse frequency, spark discharge is hard to occur and excellent corona discharge is generated. Accordingly, they have disclosed this type corona discharge processing, which comprises the use of a high voltage pulse, in the Japanese Official Patent Provisional Publication No. heisei 4-328135.
However, in the case where a high voltage pulse having a very narrow pulse width as mentioned above is used, if the corona discharge processing is performed by applying a high voltage in order to enhance the effects, the energy efficiency becomes extremely bad and therefore, an interelectrode distance, that is a distance between the electrodes, can not be large. In the processing like this, an interelectrode distance with which corona discharge processing is practically possible is in a range of from 3 to less than 5 cm and, in the case where articles having a thickness thicker than the interelectrode distance and having various three-dimensional shapes are processed, there could not be obtained any satisfactory result.
If a high voltage pulse having a pulse width of 1 .mu.s or less as in the aforementioned prior arts is used, the generation of spark discharge is effectively prevented, however, in a case of using a high voltage pulse having a narrow pulse width like the aforementioned, if the interelectrode distance becomes large, for example, 5 cm or more, it is hard to form a corona streamer (an accelerated electron stream) which is necessary for corona discharge processing, and also, even if the corona streamer is formed, the energy of effective corona discharge becomes small and the surface-processing effect on a surface of the resin forming is small.
Because of this, to afford the effects of corona discharge processing sufficiently on a surface of the resin forming, it is necessary to take the processing time in an extremely long extent. If the processing takes a long time, productivity lowers, so that it is unsuitable for industrial operation. Also, because the interelectrode distance is 5 cm or less, that is small, in a case where the resin molding has a complex three-dimensional shape, the structure of a discharge electrode becomes complex and it takes very high cost. If the discharge electrode is remade at every case where the shape of resin formings is altered even slightly, it can not be adopted in the use where the shape alteration of resin formings frequently occurs. Also, in a case where resin formings having different shapes are processed by one line of corona discharge processing, the electrode must be exchanged in every case where the resin formings are changed, and it requires very troublesome operation.